The ctenophore does not have a larval life stage and instead develops directly from embryo to adult

What is it about?

Ctenophores or “comb jellies” are gelatinous marine invertebrate animals that diverged from all other living animals very early in animal evolution and are therefore important to understand what features of animal life on earth might be universal. For about 140 years, scientists have understood ctenophores to exhibit a unique life-history pattern: two phases of sexual reproduction, separated by a period of growth and development where reproduction ceases. However, in our paper “Ctenophores are direct developers that reproduce continuously beginning very early after hatching,” we found that Mnemiopsis leidyi ctenophores do not pause their reproduction so long as adequate nutritional conditions are maintained. We were watched individual Mnemiopsis, and carefully controlled their diet, culture density and temperature to count the reproductive output of individual animals throughout their life cycle. Our new results overturn the idea that ctenophores have a special kind of life cycle. We also show that the dietary omega-3 fatty acid DHA (docosahexaenoic acid) is required for spawning in Mnemiopsis and is likely important for fertility in virtually all animals. While Mnemiopsis is native to the eastern coast of North America, it is an ecologically and economically significant invasive pest in European waters. Perhaps the most important ramification of our results is that it opens the door for researchers to maintain breeding ctenophore colonies virtually anywhere in the world and will facilitate rapid progress into the molecular understanding of embryogenesis, body plan formation, and novel cell type evolution in one of the most phylogenetically pivotal taxon on Earth. This new understanding of the Mnemiopsis life cycle will be important to ecologists and fisheries managers trying to control invasive populations, and makes clear how difficult that job might be, since Mnemiopsis can routinely reproduce at microscopic sizes and starting only 10 days after their own hatching. This result will also propel Mnemiopsis to become an even more important research organism. Our lab, at the University of Florida's Whitney Laboratory for Marine Bioscience located in St. Augustine, uses the locally abundant ctenophore Mnemiopsis leidyi (also known as the “sea walnut”), which can be seen in the daytime by the rainbows from their prismatic locomotory structures and at night from their bioluminescence. Only a few places lucky enough to have marine research facilities, like the Whitney lab, have been able to do this research because these animals are extremely delicate and very difficult to maintain in laboratory culture, making coastline access essential to collect more animals as needed. However, this new information about their life cycle will allow laboratory culture of these animals anywhere, and will allow us to maintain genetic lineages and perform multi-generational, transgenic experiments.

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Photo by Jonathan Diemel on Unsplash

Photo by Jonathan Diemel on Unsplash

Why is it important?

The practical implications for this research will be in two areas: 1. Making work with ctenophores more accessible to researchers located in many places, not just marine labs. This is important because biologists are increasingly interested in ctenophores for understanding animal regeneration following injury, basic insights into how the nervous system works, and how things like regeneration and nervous systems evolved in the first place. Knowing this will be important for knowing what is likely universal, and therefore can potentially be used to help humans. A shorter life cycle and ease of lab culture is something researchers look for in lab organisms because it enables genetic experiments. 2. While these animals are native where we are, they are invasive elsewhere, with major ecological and economic consequences, particularly for fisheries. This paper shows that they are reproductive before they would ever be visible to the naked eye, and that things like warmer temperatures and particular types of algae their prey eat greatly increase their reproductive ability. Understanding their basic reproductive biology is important to controlling these and other invasions, particularly in a warming world.

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